1. Technical Field.
The present invention relates to a radio receiver and, more particularly, relates to a radio receiver which performs clock recovery by detecting power.
2. Description of the Related Art
In a time division multiple access (TDMA) system, multiple users occupy several channels separated by time. Each user transmits and receives at specified times. Due to this time division, a method of symbol clock recovery also known as symbol synchronization is needed to correctly demodulate the receive signal. The closer the estimate to the true symbol timing, the less of a performance degradation will be seen. In previous time symbol synchronization systems, a significant time frequency offset can not be tolerated. At the time the symbol clock is recovered, a significant frequency offset is often still present on a received signal. Further, in such systems, the phase is not known, which makes the task of obtaining symbol synchronization more difficult.
A typical solution to this problem is to use a sampled waveform correlator or a type of phase-locked loop to extract the symbol timing. The sampled waveform correlator requires additional information such as a preamble and requires a significant amount of processing power. The phase-locked loop method is effective for a continuous transmission method, but in a time division multiple access (TDMA) environment, does not perform nearly as well as the sampled waveform correlator. Both techniques degrade when a frequency offset due to, for example, Doppler shifts, is introduced.
A technique for coherent demodulation of a received time division multiple access (TDMA) radio signal has been proposed by Chuang and Sollenberger in U.S. Pat. No. 4,941,155. Chuang and Sollenberger reconstruct the in-phase and quadrature signals from the differential phase of an incoming signal. A vector sum of the reconstructed inphase and quadrature signals is taken, exploiting the fact that the phases of the incoming signal add constructively only at the optimal sample. At other than the optimal sample, the samples add destructively. The demodulator of Chuang and Sollenberger will tolerate a frequency offset less than the symbol rate divided by the number of samples per symbol. A technique capable of tolerating higher frequency offset such as due to Doppler shift is needed. Furthermore, a simple technique consuming less processing time and circuitry would also be desirable.
Furthermore, another technique by Sabel and Cowley has disclosed a coherent demodulator with a frequency and phase estimation performed prior to a timing estimate. Such frequency and phase estimates ensure that the signal input to a squaring or magnitude operation has no frequency offset and near perfect phase synchronization. Sabel and Cowley do not provide for a systems such as quadrature phase shift keyed systems (QPSK) having complex modulation. A simpler technique requiring processing of less data is desired. Further, a system capable of tolerating high frequency offsets or errors is needed. These frequency offsets or errors could be due to large Doppler offsets or frequency oscillator errors.